1. Field of the Invention
The present invention relates to a method and an apparatus, for setting a coordinate system (in particular, a conveyor coordinate system). The present invention also relates to a robot system provided with a coordinate system setting apparatus.
2. Description of the Related Art
A system configured so that a sensor (e.g., a vision sensor) detects a position of an article conveyed by a conveyor and a robot picks up the article from the conveyor based on the detected position, has been known. Further, a system configured so that a sensor (e.g., a phototube sensor) detects a position of an article tray conveyed by a conveyor and a robot places an article on the article tray based on the detected position, has also been known. In this type of robot system, in general, a coordinate system (in the present application, referred to as a “conveyor coordinate system”) having a predetermined relationship with a base coordinate system of a robot is set as a coordinate system provided for a conveyor for defining a position of an article and/or an article tray (in the present application, referred to generically as an “object”) on the conveyor.
For example, Japanese Unexamined Patent Publication No. H9-131683 A (JP H9-131683 A) discloses a vision sensor and robot system configured so that a vision sensor recognizes the positions of a plurality of workpieces conveyed by a conveyor and each of a plurality of robots picks up a workpiece by a tracking operation based on a recognized position. In this system, a tracking coordinate system configured to be displaced at a speed equal to and a direction identical to those of the conveyor is set as a coordinate system for defining the position of the workpiece on the conveyor. The tracking coordinate system is provided, at an initial position, with the origin spaced upstream from the base coordinate system of the robot by a predetermined distance, and is set in such a manner that its X-axis coincides with the running direction of the conveyor.
On the other hand, in a robot system including a robot and a conveyor, as a result of a supplementary work performed for the robot system which has already been established, such as maintenance on the robot and/or conveyor, relocation of the entire system to another place, etc., sometimes a positional relationship between the robot and the conveyor changes between before and after performing the supplementary work. If the positional relationship between the robot and the conveyor changes, it is necessary to correct an operation taught point which has been taught to the robot before performing the supplementary work, in accordance with the change of the positional relationship after performing the supplementary work. For example, Japanese Patent No. 3733364 B (JP 3733364 B) discloses a method of correcting a taught position, configured so that a vision sensor measures three-dimensional positions of at least three points defined on an object to be worked (i.e., a workpiece) or on a holder thereof, before and after the relocation of the robot system is performed, and positions of taught points provided in a robot operation program, which have been taught before performing the relocation, are corrected, based on the measured positional data, so as to compensate for a change in a relative position of the robot to the workpiece, which has been caused between before and after performing the relocation.